1. Field of the Invention
The present invention relates to an electrolyte, and more particularly to a composition of nano-tube composite polymer electrolyte and a fabrication method thereof.
2. Description of the Related Art
In 1973, Wright et al. mixed polyethylene oxide (PEO) and KSCN for forming a crystal complex. In 1975, they further proved that the complex had a conductivity higher than 10−4 S/cm at high temperature (>100° C.). PEO salt could have conductivity about 10−3 S/cm similar to that of organic electrolyte solution at 100° C. Then, more research and study were undertaken for improving the conductivity of the polymer electrolyte at room temperature and achieving practical application.
Generally, the polymer electrolyte is made of a polymer substrate and a metal salt. In the amorphous region of the polymer substrate, the metal salt interacts with functional groups of the polymer so that the metal salt moves along the chains of the polymer for transmitting the metal ions.
For the time being, the systems of polymer electrolyte includes Polyethylene Oxide (PEO), Polyvinylidene Fluoride (PVDF), Poly(methyl Methacrylate) (PMMA), Polyvinylidene chloride (PVC), Polyacrylonitrile (PAN), etc. The cations of the metal salt can be, for example, Li, Na, K, Mg, etc; the anions can be BF4, SCN, SO3CF3, AsF6, PF6, N(CF3SO3), etc. The bigger anion group has higher delocalized charges and easily ionizes.
Generally, the conductivity and the mechanical properties of the electrolyte can be improved by adding organic material for inhibiting the crystallization of the polymer, reducing Tg, enhancing the mobility of ions. It also can be achieved by selecting a proper salt with effectively dissociation property for enhancing the mobility of ions in the electrolyte. U.S. Pat. No. 5,643,490 disclosed a method of manufacturing a polymer containing tetraalkylammonium. U.S. Pat. No. 5,688,613 disclosed a method of doping-polybenzoimidazole with H3PO3. U.S. Pat. Nos. 5,581,394, 5,705,084, 5,645,960, 5,731,104, 5,609,974 and 5,586,001 disclosed a method of adding organic plasticizer such as Ethylene Carbonate (EC), Propylene Carbonate (PC) or Dimethylcarbonate (DMC) into polymer electrolytes. Although these methods can improve the conductivity of the solid-state electrolyte at room temperature, they will crate other problems. Under high temperature and pressure, the electrolyte has bad thermal stability, softens and causes circuit shortage within cells.
The modifier can be inorganic material, such as nano-particle TiO2 in addition to organic material. Although the inorganic modifier can improve the mechanical properties of the polymer electrolyte, form a good interface between electrodes and the electrolyte, reduce the sediment of Li ions and extend the service life of battery, the conductivity and distribution of the electrolyte are not desirable at room temperature.